Hybrid vehicles have been well known that are driven by engines (internal combustion engines) and motors (electric motors) used in conjunction with each other. The proportion of power that each driving source of a hybrid vehicle distributes is variably controlled depending on the output characteristics of the installed engine and motor and the running states of the vehicle. For example, a vehicle equipped with a mild hybrid system can operate in different modes, such as an engine-only mode during which the vehicle runs solely on the engine depending on the running state of the vehicle, a power regeneration mode, and an assist mode during which the driving force generated at the motor is used to assist the driving force generated at the engine. A mild hybrid system adopts a motor which owns relatively low output performance and achieves superior fuel economy (fuel saving) compared to vehicles other than hybrid vehicles.
Vehicles taking a strong hybrid system have an additional motor-only mode during which the vehicle runs solely on the motor. That is, the motor is operated while the engine is shut down to run the vehicle fully on electricity. Strong hybrid vehicles achieve high fuel economy compared to mild hybrid vehicles but the installment of high-power motors and corresponding batteries are adopted.
Such hybrid vehicles taking multifunctional hybrid systems have powertrains with increasingly complicated structures. A hybrid vehicle is equipped with various devices to establish an energy path appropriate for the driving mode, such as a power generator, a motor, a transmission for varying the torque and rotational speed of the driving wheels, a clutch for switching the driving source, and a driving unit for driving the clutch.
Further reductions in size and weight of powertrain systems are essential for improvements in environmental adaptability and fuel economy of vehicles. In particular, the mountability of powertrains in vehicles taking a strong hybrid system tends to be low due to large motors and generators; thus, reductions in sizes of the powertrains are significantly important.
To solve such issues, an approach has been proposed for enhancing the mountability of a powertrain by aligning the motor and the generator along an axis. For example, PTL 1 (Patent Literature 1) describes a driving unit of a hybrid vehicle equipped with coaxially positioned motor and generator having a same outer diameter. Such an arrangement reduces the side shape (side surface area in appearance) of the powertrain and enhances the mountability.